Freckle Formation Tendency of a Single-Crystal Nickel-based Superalloy with Minor Carbon Additions

The freckle formation tendencies of single-crystal superalloys with minor amounts of carbon were investigated. Carbon addition resulted in TaC-type carbide (MC) precipitation earlier than the eutectic phase in the interdendritic region, which induced Ta depletion and resulted in increases in the W and Re contents in the eutectic. The weakened density imbalance between the interdendritic region liquid in the mushy zone and the melted bulk liquid prohibits convection flow. Additionally, as the amount of carbon intentionally added to the superalloy increases, the volume fraction of the eutectic in the superalloy decreases, and the MC changes from a blocky to script-like shape in the interdendritic region. A lower volume fraction of the eutectic phase and the script-like carbide network can depress thermosolutal convection in the interdendritic region in the longitudinal direction. For these reasons, the freckle formation tendency decreased. As the carbon content of superalloys increases, the Rayleigh number decreases, resulting in a decrease in freckle formation. The Rayleigh criterion can be used to evaluate freckle formation tendency.